1. Field of the Invention
The present invention relates to a motor controller that, with high efficiency, performs weakening-field control on motors such as those used in electric automobiles.
2. Description of the Related Art
The brushless motor, which has no mechanical commutation mechanism, controls the sir current that flows through the stator windings, synchronously with the rotated position of the rotor, thus rotating the rotor in the predetermined direction, and generating the predetermined output torque. Because the magnetic field is formed by permanent magnets, this motor is very efficient, and is widely used in electric automobiles.
After describing brushless motor characteristics and weakening-field control, we will describe an example of the prior art.
Permanent magnets are set in place in the rotor of a brushless motor for forming magnetic fields. Let us posit a d axis that is in the same direction as the magnetic field, and a q axis rotated 90.degree. away from the d axis by an electrical angle of 90.degree.. Now, the q axis will be in the same direction as the back voltage excited in the stator winding by the magnetic field, and the current phase will advance when a current is added in the direction of the d axis. Hereinafter, as in FIG. 17, the size of the current is called I, the current component flowing in the d axis direction is called the d axis current Id, the current component flowing in the q axis direction is called the q axis current Iq, and the current phase is denoted as .beta..
When a current flows in the stator winding, the q axis component of the terminal voltage, Vq, and the d axis component thereof, Vd, can be expressed as in Eq 1. And the brushless motor output torque Trq may be expressed as in Eq 2. In these formulas, .omega..theta. is the electrical angular speed, R is the stator winding resistance, .psi. is the armature interline flux number, Ld is the d inductance, and Lq is the q axis inductance. ##EQU1##
In a surface-magnet type of brushless motor in which the permanent magnets are positioned on the surface, the d axis inductance and q axis inductance are equal (Ld=Lq), so the second term in Eq 2 will be 0, and, when the current value I is constant, the output torque Trq will have a maximum value at current phase .beta.=0.degree.. In an embedded-magnet type of motor, on the other hand, wherein the permanent magnets are embedded in the rotor, the d axis inductance Ld is smaller than the q axis inductance Lq, so a maximum value will be obtained in the first term in Eq 2 when the current phase .beta.=0.degree., and in the second term in Eq 2 when .beta.=45.degree.. Accordingly, as plotted in FIG. 18, a maximum value T0 for the output torque is expressed at some value (.beta.0.degree.) between .beta.=0.degree. and .beta.=45.degree..
When a brushless motor is running, if the rotational speed a of the brushless motor is increased, the induced voltage arm becomes larger, as represented in the weakening-field control vector diagram in FIG. 19A. When the voltage V to which .omega..sub.s .psi., RIq, and .omega.LqIq have been vector-added reaches the voltage limit circle, it will no longer be possible to increase the rotational speed above the brushless motor rotational sped a attained when the voltage V reached the voltage limit circle.
Now, when the power supply is something like a battery, due to the load current, the battery's terminal voltage and current values will vary. For the sake of simplicity, however, the battery terminal voltage (i.e. the radius of the voltage limit circle) is here assumed to be constant.
Let us next consider what happens when the brushless motor rotational speed is increased. As diagrammed in FIG. 19, by having a current Id flow, a voltage .omega.LdId will be generated in a direction returning back inside the voltage limit circle. In this way, excess voltage is generated that increases the rotational speed of the brushless motor as diagrammed In FIG. 19B. When the rotational speed of the brushless motor is constant, it is possible to produce a q axis current Iq by the amount of the excess voltage generated, making it possible to generate more output torque in the brushless motor. And when the current value is constant, as diagrammed in FIG. 19C, it is possible to increase the rotational speed .omega. by the amount of the excess voltage generated. Controlling a motor by making a d axis current Id flow in the stator winding and generating a voltage excess is called weakening-field control.
Now, if the current value is held constant and the current phase is varied, the d axis current Id will increase, so a voltage excess will be generated and weakening-field control will be effective. At the same time, the q axis current Iq will be diminished, causing LqIq to decease and the voltage value V to become smaller, thus generating further voltage excess.
Furthermore, the d axis current Id that generates the excess in the voltage need only be the minimum necessary to make the terminal voltage supplied to the brushless motor return back inside the voltage limit circle. If the d axis current Id provided is greater than this necessary minimum, copper losses will increase, and the brushless motor's efficiency will deteriorate.
When there is excess voltage available, moreover, the stator current will follow the stator current command well, as diagrammed in FIG. 5A, but when the voltage excess disappears, the stator current will not follow the stator current command, as diagrammed in FIG. 5B.
One method for implementing weakening-field control for brushless motors known in the prior art is that set forth in IEA-92-30, a document of the Electrical Engineering Research Society prepared by its Industrial Electric Power Applications Research Group.
In performing weakening-field control, the d axis current command Id* is calculated as in Eq 3. ##EQU2##
Here, .omega..sub.base, is the base rotational speed, .omega..sub.max is the maximum rotational speed, and Id*.sub.max is the d axis current at the maximum rotational speed .omega..sub.mar.
In the 1991 Society of Electrical Engineers Industrial Applications Division National Convention Monograph Collection No. 74, on pp.310-315, the d axis current command Id* for performing weakening-field control is calculated using the target rotational speed, d axis winding reactance, q axis winding reactance, aor winding resistance, and no-load induced voltage at the unit speed.